Fix (at least for me):
I too had this problem after updating both my Win7 64-bit Shuttle XPC and my mid-2011 MacBook Air w/ iTunes 11.0
I recently bought an iPad and an AppleTV v3 in attempt to listen to and control the same symultaneous music throughout my apartment. Before the update, it kinda worked. Either it was slow/rebuffered-often or it did not work at all. Then with the 11.0 update, nothing on any of my 4 "home sharing" electronics. I was contemplating just buying an Airport Express. After reading this post, I decided to scrub thru all the firmware settings of my 2010 Linksys E2000 router. I had "Filter NAT Redirection" and "Filter Multicast" already disabled, so after reading on it, I disabled "SPI Firewall Protection" and all my problems immediately went away.
I'm now able to control two computers' library's to two separate speaker systems all from my iPad, with minimal wifi delay.
Keep in mind I am on a personal encrypted network in an apartment. Only myself and my 8-ish devices reside on it, so nothing and noone (local) is getting past my WPA2.
I hope this helps.